A test for the atomic_ref deduction guide

Summary:
[Folly] A test for the `atomic_ref` deduction guide, which permits removal of the preprocessor guard in the definition of `make_atomic_ref`.

Some parts of folly still support legacy versions of C++ but we do not necessarily run the tests on legacy platforms.

Reviewed By: akrieger

Differential Revision: D22635366

fbshipit-source-id: 8961d3465b63875a660d9e83d094ebf0cd64d1df

folly/synchronization/AtomicRef.h

@@ -140,9 +140,6 @@ struct make_atomic_ref_t {
               alignof(T) == alignof(std::atomic<T>),
           int> = 0>
   atomic_ref<T> operator()(T& ref) const {
-#if __cpp_deduction_guides >= 201703
-    return atomic_ref{ref};
-#endif
     return atomic_ref<T>{ref};
   }
 };

folly/synchronization/test/AtomicRefTest.cpp

@@ -20,6 +20,12 @@
 
 class AtomicRefTest : public testing::Test {};
 
+TEST_F(AtomicRefTest, deduction) {
+  long value = 17;
+  auto ref = folly::atomic_ref{value}; // use deduction guide
+  EXPECT_EQ(17, ref.load(std::memory_order_relaxed));
+}
+
 TEST_F(AtomicRefTest, integer) {
   {
     long value = 17;